Optical filters are key components in a wide variety of optical systems including optical telecommunications, optical displays and optical data storage. An optical filter is used to selectively reflect or transmit light of a predetermined wavelength. Typical uses include channel selection in wavelength division multiplexed (WDM) systems, multiplexers, demultiplexers, switches and wavelength selective laser cavity reflectors.
Subwavelength resonant grating filters (SRGFs) are highly promising for many optical filter applications. SRGFs typically comprise a linear array of grating lines overlying an optical waveguide. The spacing between successive grating lines is smaller than the wavelength of the light they process, hence they are called subwavelength gratings. They are highly reflective for light of a specific wavelength that resonates with the spaced grating lines. Further details concerning such filters can be found, for example, in U.S. Pat. No. 5,216,680 issued to Magnusson et al. on Jan. 1, 1993 and U.S. Pat. No. 5,598,300 issued to Magnusson et al. on Jan. 28, 1997, which are incorporated herein by reference.
While SRGF's are compact and highly reflective, they are typically fixed in resonant wavelength at their fabrication. Efforts have been made provide a tunable SRGF by disposing an electro-optic medium adjacent the grating. See U.S. Pat. No. 6,215,928 issued to A. Friesem et al. on Apr. 10, 2001. Unfortunately the Friessem et al structures do not perform sufficiently well for practical application. Accordingly, there is a need for an improved tunable subwavelength resonant grating filter.